Typically, a device for focusing a laser light beam comprises an emission source of the laser light beam along an optical emission path and an optical focusing system, typically comprising at least one focusing lens or mirror, adapted to focus the laser light beam at a predetermined focusing distance.
As known, the maximum resolution of an optical code reader, that is, the minimum dimension of the details that can be detected thereby, depends on the beam diameter at the distance at which such beam is focused.
For a wide range of applications, the distance at which the optical code is located, and therefore the distance at which the light beam has to be focused, is not predetermined. It is therefore necessary to use devices capable of automatically adjusting the focusing distance. Such devices allow extending the area within which the maximum resolution can be attained. Such devices are also known as auto-focus devices.
In conventional auto-focus devices, the automatic adjustment of the wanted focusing distance is achieved through the relative mechanical movement of the light source and of the optical focusing system or, in the case of focal variation optical devices, of a portion of the optical focusing system.
Devices are known wherein such mechanical movement is actuated on the basis of a signal detected through special detection means and representative of the distance at which the support containing the information to be read is located.
For example, U.S. Pat. Nos. 4,604,739 and 5,446,710 disclose auto-focus devices for readers of information contained on a reflecting support, wherein the wanted focusing distance is adjusted on the basis of a signal indicative of the distance of the reflecting support. Such distance is calculated by intercepting and detecting, through a lateral interferometer, the light beam reflected by the reflecting support.
U.S. Pat. No. 4,641,020 discloses an auto-focus device for a reader of information contained on a reflecting support, wherein the wanted focusing distance is adjusted on the basis of a signal representative of the position of the focusing lens relative to the emitting source.
U.S. Pat. No. 6,134,199 discloses an auto-focus device for a reader of information contained on a reflecting support, wherein the wanted focusing distance is adjusted on the basis of a signal representative of the distance of the reflecting support. Such distance is calculated by detecting the light beam reflected by the support and re-collimated through the same focusing lens arranged in the optical emission path.
U.S. Pat. No. 6,621,060 B1 discloses an auto-focus device for mechanical laser processing, wherein the focus correction is actuated through the observation of the spot produced by an accessory laser light beam on the surface of the piece to be processed through the focusing lens of the laser light beam used for the mechanical processing.
U.S. Pat. No. 6,728,171 B2 discloses an auto-focus device for imaging apparatus, wherein the focus correction is actuated through an accessory light beam focused on the surface of the focusing lens and observed by a sensor always through the same lens.
In some of the auto-focus devices described in the patents mentioned above, the focus position is adjusted through the measurement of the reciprocal position between the surface on which the focus has to be maintained and the focus itself.
The Applicant has found that some of such devices have the disadvantage of being usable only if the information is contained on a reflecting support, a case that happens in a limited class of applications.
In other currently known devices, the focus position is adjusted indirectly through the measurement of the reciprocal position of the optical focusing system relative to the emitting source, or in the case of focal variation optical devices, of the reciprocal position of some elements of the optical focusing system relative to the entire optical system.
The Applicant has found that such devices have the disadvantage of being very sensitive to possible errors in the relative positioning between optical focusing system and light source or, in the case of focal variation optical devices, between elements of the optical focusing system. In particular, the Applicant has found that in such devices, possible positioning errors or unwanted movements of the mechanical and optical parts of the optical focusing system, resulting for example from thermal expansion, mechanical clearance or vibrations, cause an error in the adjustment of the expected and/or wanted focusing distance. In other words, the Applicant has verified that the above errors (or unwanted movements) cause the positioning of the focusing point at a focusing distance other than that expected and/or wanted.
In fact, by simplifying the description of the optical systems through the paraxial approximation, position q of the focusing point is given by:
      1    q    =            1      f        -          1      p      wherein f is the focal length of the optical system used for focusing and p is the position of the light source relative to the optical system.
Therefore the absolute error Δq associated with the focusing point is:
      Δ    ⁢                  ⁢    q    =                                                  f            2                                              (                              p                -                f                            )                        2                                      ⁢      Δ      ⁢                          ⁢      p        +                                                p            2                                              (                              p                -                f                            )                        2                                      ⁢      Δ      ⁢                          ⁢      f      with Δp absolute error on the positioning of the optical system and Δf absolute error on the focal length of the optical system; in particular, Δf is other than zero in the optical systems based on the actuation through focal variation.
From the above equation it is clear that errors Δp and Δf cause an error of the focusing point Δq.
Often, errors Δp and Δf are not systematic, as they depend on factors such as temperature, mechanical clearance, non-repeatability of the construction parameters of the optical, electronic or mechanical components. An indetermination of the actual position of the focusing point therefore occurs; in current focusing devices, such indetermination forces to the use of light beams of a larger diameter, with consequent reduction of the maximum resolution.
The Applicant has further found that in the focusing devices discussed above, the operations for assembling and calibrating the mechanical and optical components are quite difficult, as it is necessary to ensure strict tightening tolerances between fixed components and moving components. This implies the use of particularly sophisticated construction solutions, with obvious consequences both on the final cost of the device and on the dimensions of the same. Similar problems arise with reference to the device used for the mechanical moving of the focusing systems: as such device has to ensure movements in the range of a millimeter or of a tenth of millimeter, it must be highly accurate.
U.S. Pat. No. 6,119,942 discloses an auto-focus device used in an optical code reader through the scanning of a laser light beam, wherein the focus correction is determined by the measurement of the diameter of the laser light beam at a predetermined distance, this measurement being obtained by scanning the laser light beam on a photosensitive element and subsequently measuring the scanning duration.
The Applicant has found that in such device, the direct adjustment of the focusing distance by measuring the beam diameter through the scanning on a photosensitive element is subject to considerable inaccuracy, since the variation of the beam diameter is weakly related to the focusing distance. In particular, such variation tends to zero as the focusing distance approaches the emission point, where greater focusing accuracy is typically required. In fact, at distances closer to the reader, the capability of reading higher resolution codes is typically required and therefore, a high focusing accuracy becomes important. The Applicant has further found that in such device the adjustment of the focusing distance cannot be carried out in real time and continuously during the scanning due to the fact that the diameter detecting system is arranged downstream of the scanning system and in a marginal position of the scanning field. The measurement of the diameter, and therefore the adjustment of the focusing distance, can be carried out only once for each scanning. Such adjustment system, therefore, is not capable of adapting the focusing distance in an accurate and reliable manner in situations wherein the support containing the code is curved or in other situations wherein a continuous and precise adjustment of the focusing may be necessary.
The Applicant, therefore, has considered the problem of providing an auto-focus device which should be small sized, not expensive, fast, with a high resolution and accuracy, and which should be insensitive to any positioning errors or unwanted movements between the mechanical and optical parts of the optical focusing system and/or to errors on the determination of the focal length of the optical focusing system, so as to obtain an accurate and reliable and, if needed, continuous and real time automatic adjustment of the focusing distance.